The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Rare earth elements are of increasing economic importance as the highly specialized properties of individual elements find applications in a wide range of pervasive products. For example, lanthanum is a component of catalysts employed in oil refineries, neodymium is a component of solid state lasers, neodymium magnets are widely employed in headphones, hard disk drives, and DVD and CD-ROM drives, and europium and terbium are dopants in primary color phosphors for plasma and LCD screens.
At present, the principal source of most rare earth elements is China, and the principal deposits are in the Sichuan and Baotou regions.
With limited prospects for expanded production, and diversion of an increasing proportion to domestic customers, China is unlikely to be able to meet the expected increase in world demand for rare earth elements over the coming years. Since the gap will not be met by traditional sources outside China, such as monazite deposits for certain of the elements, there is a growing requirement to tap non-traditional ores containing rare earth elements. One of these is a composite ore containing rare earth elements that includes a monazite group mineral and an apatite mineral. Apatite is a phosphate of calcium, while a “monazite group mineral” is a composite monoclinic phosphate of rare earth elements and several other substituents.
When a composite ore of this structure has been subjected to conventional or other known processing regimes, it has proven difficult to convert the ore to an economically useful deposit. High thorium content in both principal components of the ore have proven a hurdle to economic conversion of the phosphate base of the apatite to a “clean” fertilizer product, while the uranium content associated with the thorium cannot be economically extracted in sufficiently pure form.
Primary treatments that were attempted included a nitric acid leach, a hydrochloric acid leach, a sulphuric acid bake/water leach and a caustic crack. It was found that direct treatment of the ore resulted in either low rare earth recovery, or poor leach selectivity for rare earths over calcium and phosphorous.
It is an object of the invention to provide a process for recovering rare earth elements from composite ores containing rare earth elements that include a monazite group mineral and an apatite mineral, preferably in a manner that also permits economic extraction of the phosphate values of the ore and removal of thorium to an extent that can meet end-product specifications.